Sheet-like optical articles include, for example, Fresnel lens sheets used for a projection screen of a projection television and the like, lenticular lens sheets and prism sheets used for a backlight of a liquid crystal display device. These articles generally have a structure in which a lens part made of a cured resin material is formed on the surface of a sheet-shaped transparent substrate. For example, as described in Japanese Patent Laid-Open No. 2000-297246, a prism sheet is produced by a process comprising casting an active energy ray curable resin between a sheet-shaped transparent substrate such as a transparent plastic sheet and a cylindrical lens mold having a lens pattern on the surface and curing the resin composition by irradiating the resin composition with an active energy ray from the side of the sheet while transporting the sheet, thereby continuously forming prisms. In Japanese Patent Laid-Open No. 2000-297246, a composition using hexafunctional urethane (meth)acrylate and a di(meth)acrylate compound in combination is proposed in Examples as an active energy ray curable resin for a prism sheet.
The resin composition described in the publication has good scratch resistance and heat resistance when formed into a prism sheet. However, the prism sheet tends to be warped at four corners when put on a level position with the prism face upward. For this reason, the sheet has a problem that handling of the sheet using a machine and determining the position to put the prism sheet in handling is difficult in the step of assembling the sheet in a backlight. Moreover, warpage of the sheet tends to become even greater when put in a high temperature condition for more than a given time, sometimes affecting image qualities.